Smoke detector

ABSTRACT

A smoke monitoring instrument having first and second chambers, each with a respective photodetector means, a light source common to both chambers and positioned to illuminate part of the interior of each chamber, means for conveying a first air sample from the space to be monitored into said part of the first chamber, means for conveying a second air sample from outside the space into said part of the second chamber, each photodetector means being arranged to receive light from the source which is scattered by its respective air sample but not to receive light directly from the source whereby the effects of suspended material in the first sample originating from outside the space to be monitored can be observed or removed from the output of the instrument.

United States Patent 1 Packham et al.

[ 1 Apr. 1,1975

[ SMOKE DETECTOR [75] Inventors: David Roy Packham, Upper Be'aconsfield,Victoria; Leonard Gibson, Frankston, Victoria, both of Australia [22]Filed: June 5, 1973 [21] Appl. No.: 367,260

[30] Foreign Application Priority Data June 6, 1972 Australia 9230/72[52] US. Cl 356/104, 250/574, 250/575, 340/237 S, 356/207 [51] Int. Cl.G0lm 21/00, G01m 21/12 [58] Field of Search 356/104, 103, 207, 211,356/212; 340/237 S. 250/574, 575

3,231,748 1/1966 Hacssler et al. 3,313,946 4/1967 Goodwin et al.3,563,661 2/1971 Charlson et al 356/104 Prinutry E.\'aminerVincent P.McGraw Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak[57] ABSTRACT A smoke monitoring instrument having first and secondchambers, each with a respective photodetector means, a light sourcecommon to both chambers and positioned to illuminate part of theinterior of each chamber, means for conveying a first air sample fromthe space to be monitored into said part of the first chamber, means forconveying a second air sample from outside the space into said part ofthe second chamber, each photodetector means being arranged to receivelight from the source which is scattered by its respective air samplebut not to receive light directly from the source whereby the effects ofsuspended material in the first sample originating from outside thespace to be monitored can be observed or removed [56] References Citedfrom the output of the instrument.

UNITED STATES PATENTS 2,966,092 12/1960 Hartridgc 356/207 20 Claims, 7Drawing Figures 71 l 72 7) 12 74 5 7 76 65 66 61 64 60 7 1 1 I 1 1 -I 11 l I l l i l l l l j l PATENTEU 1 1975 874. 79 5 CLOCK PULSE GENERATORAMPLIFIER w F I (PHOTOMULTIPLIER l I 2. 1.? 1a

\ L INTEGRATOR AND HOLD CIRCUIT 7 EQ m T IJ B E I 51 U2 V UF R DlFF.fRECORDER AMP. j L. 33. 3; is

SMOKE DETECTOR This invention relates to early warning fire detectors,and has for its object the provision of means for detecting the presenceof smoke particles (or similar light scattering material for example.airborne dust hereafter included within the term smo.ke") in gases suchas air.

Existing early warning fire detection systems employ smoke sensing unitswhich operate in a variety of ways, chiefly by ionization, lightobscuration or light scattering. A common characteristic of these unitsis that they must be designed and operated in such a way that falsealarms are minimized. They are intended for connection into automaticalarm systems. and false alarms can result in substantial inconvenienceand occasionally even water damage to property which they are intendedto protect. In practice. this requirement results in the setting of suchunits to be relatively insensitive, and substantial concentrations ofsmoke must be present to trigger them. For example. the specificationsof one known device require it to trigger only at a light obscurationfigure of percent per meter, and to remain quiescent at an obscurationfigure of4 percent per meter. With the type of fire likely to develop intelephone exchanges, computer installations or the like, smokeconcentrations of this order take a substantial time to accumulate, sothat potentially valuable early warning time is presently being lost.However, merely increasing the sensitivity of the units is not asatisfactory solution to the problem of providing a very early firewarning because of the accompanying rising incidence of false alarms.Such false alarms may be triggered either by instability in the units,or by smoke not associated with a fire hazard for example, cigarettesmoke within the area being monitored. or high outside air pollutionlevels. The invention has for its object the provision of means whichcan provide an earlier fire warning than is customary with the describedknown systems.

ln one form, the invention provides a smoke monitoring instrument havingfirst and second chambers, each with a respective photodetector means, alight source common to both chambers and positioned to illuminate partof the interior of each chamber means for conveying a first air samplefrom the space to be monitored into said part of the first chamber,means for conveying a second air sample from outside the space into saidpart of the second chamber, each photodetector means being arranged toreceive light from the source which is scattered by its respective airsample but not to receive light directly from the source. The lightsource is preferably a flash tube operated periodically to illuminatethe air samples with an intense flash of light, and the photodetectormeans are preferably photomultiplier tubes.

The source of light may be continuous rather than intermittent but thelatter is preferred because less energy is required to power the lampand less heat is developed in the lamp. In addition, advantage may betaken of the period when the source is inactive to monitor noise levelsin the instrument so that more accurate output is possible withappropriate compensation.

It is particularly advantageous to use a single flash tube rather than aseparate tube for each chamber because the light output from lamps isinclined to vary from one pulse of light to another and thus at oneflashing instant the sources produce different amounts of light whichwould lead to errors. The use of a single flash tube eliminates thispossible source of error. There still remains however, the possibleerror due to fluctuations in the output of the single source but thiscan be removed by taking an output from the difference of the outputs ofthe photodetector means. Further, it has been found that a flash tube ofrelatively low capacity and operated near its full capacity will produceoutput pulses which are more uniform in intensity than those obtainedfrom a higher capacity flash tube operated at low capacity.

We have found that various normal" causes of smoke in the air of a spaceto be monitored for fires exhibit characteristics over a period of timedifferent from those associated with real fires, and accordingly, in aparticularly advantageous embodiment of the invention, the outputs fromthe detectors are recorded directly, or subtracted one from the otherand the difference signal recorded. The record should be such thatshort-interval peaks of smoke, typical of cigarette smoke or the likecan be distinguished from the steady build-up of smoke which accompaniesthe outbreak of a serious fire. With such a record, it is possible toset the sensitivity of the instrument to detect smoke levels well belowthose typical of conventional level-triggered devices. An alert may beset to trigger at light obscurations as low as 0.02 percent per meter. Asupervisor may then investigate the incident by inspecting the recordfor instance, in the form of traces on a chart from a pen recorder andtake appropriate action. Alternatively. automatic circuitry may be setinto operation to interpret the record suitably in the form of storedcharges on capacitors or the like.

A particularly advantageous feature of the instrument according to thisaspect of the invention resides in its ability to allow for the effectsof outside air pollution. Rising air pollution by smoke, which may occurduring the morning in industrialized localities, or be caused by bushfires, can bring the scattering coefficient of outside air into therange where the instrument will signal an alert. Outside air, for thepurposes of this specification, refers to the air used to ventilate thespace being monitored, and will normally be the open air surrounding thebuilding. While smoke is building up in the outside air, it will in theabsence of a fire inside have a larger light scattering coefficient thanthe inside air. When the outside smoke level drops again, however, smokewill have accumulated inside, and will register as having a scatteringcoefficient relative to the now cleaner outside air sufficiently high totrigger an alert. But reference to the record, which will show aconsiderable period during which the outside air had a higher scatteringcoefficient than the inside air, will explain the alert; and unlessextra smoke is being generated inside the space to be monitored, pushingthe difference signal above the level appropriate for the effects ofexternal air pollution, no action need be taken on the alert.

The instrument is particularly effective if it is coupled into a forcedair circulation or conditioning system. In this case, the first airsample is drawn from the exhaust air duct of the system,- and the secondair sample directly from outside, or from the inlet air duct of thesystem. The instrument may be made sufficiently sensitive that, despitethe diluting effect of large volumes of air passing into the exhaustduct, small quantities of smoke The invention also provides a method ofdetecting the generation of smoke associated with a fire hazard in aspace to be monitored. comprising the steps of drawing a first sample ofair from the space to be moni tored. drawing a second sample of air fromoutside said space. exposing both samples to a source of light andgenerating two signals, each related to the amount of light scattered bya respective one of said samples. and storing a record of said signalsover a period of time.

Preferred features of the method according to the invention will becomeapparent from the following description of particular embodiments of theinvention, in which:

FIG. 1 is a diagrammatic representation of one form of the instrumentaccording to the invention;

FIG. 2 shows a cross-section of the instrument at 22;

FIG. 3 is a diagrammatic representation of another form of theinstrument according to the invention;

FIG. 4 is an airflow diagram showing how the instrument is connected tothe space to be monitored;

FIG. 5 is a block diagram of a circuitfor operating the instrument;

FIG. 6 is a more detailed diagram of part of the de tecting circuitshown in FIG. 5;

FIG. 7 is a graphical representation showing various features of theoutput from the instrument.

The instrument shown in FIG. 1 comprises two lighttight chambers. 1 and1', the internal surfaces of which are coated with optical black paintor the like to minimize reflections. Inside each chamber are located aseries of knife edge light baffles. 2-9 and 2'-9', which restrict thefield of view of photomultipliers 10,10. to volumes 1, 1'. The lightbaffles are also coated with optical black paint to limit reflections.and have central apertures graded upwardly from baffles 3-8 (38).Baffles 2, 2' have apertures the same size as baffles 3, 3', andbaffles9, 9, have smaller apertures than baffles 8, 8', to formrespective light-traps at the end of each chamber.

FIG. 2 shows baffles 6 and 6' in a cross-section of the instrument takenat 2-2 in FIG. 1. Central apertures 11, 11 define the fields of view ofthe photodetectors l0, l. and allow air to pass from inlets 12, 12' tooutlets 13, 13. Further apertures 14, 14 in light baffles 6 and 6' areprovided for extra air circulation to allow a more even distribution ofair within the chambers. Similar air circulation apertures are providedin light baffles 5, 7 and 7'. The light baffles may be replaced whollyor in part by one or more tubes positioned to define similar fields ofview for the light detector. Xenon flash tube 15 is provided in a commonwall 16 between the chambers in such a way that flashes of light,transmitted through light diffusing windows 17, 17, for example made ofopal glass, can be scattered by smoke in the air samples in bothchambers into respective photodetectors 10, 10'. Casing 18, also incommon wall 16, houses a capacitor bank which in operation dischargesthrough lamp 15.

The instrument shown in FIG. 3 is essentially the same in operation asthat described previously but its physical arrangement and constructioninclude a number of features to improve performance and reducemanufacturing and maintenance costs. Parts in this form which are commonto that described previously 1 have the same reference numerals.

The two chambers 11, 11 are located on opposite sides of a plate 50which forms the upper part of a housing for the instrument. The upperpart of the housing with the chambers attached can be readily removed toallow access to the flash tube 15 and photodetectors 10, 10 forreplacement. The chambers 11 and 11 are formed by sheet metal structuresabout 55 cm in s length. 5 cm across, and 8 cm in depth. Both the insideand outside of the structures are painted with optical black paint. Eachchamber, 11, 11 includes baffles 51,1

51; 52, 52; 53, 53; 54, 54'; 55, 55; 56, 56'; 61, 61' extendingtransversely across the chamber 11, 11. The baffles 51, 51 and 53, 53are provided with centralaperatures 57, 57 and 58, 58 which define thefield .of

view of the photodetectorlO, 10' as shown by lines59, i

59. Light from the tube 15 enters the chambers'11,111 through openings60, 60. formed in the chambers, the

openings 60, 60 being surrounded by short branch sec- J tions 61, 61which project from the sheet metal structures defining the chambers 11,11". The branch sec-- tions are terminated by diffusing windows 19, 19

which each comprise a pair of spaced opal glass plates 7 62, 62 and 63,63, normals to the planes of which make angles of about 40 to thelongitudinal axes of the chambers 11, 11. The spaced plates are found toprovide a better source of diffused light than a single plate or asingle plate of equivalent thickness. The majority of light emanatingfrom the plates 62, 62. is inclined relative to the longitudinal axis ofthe chambers 11, 11. and more scattered light for a given smokeconcentration can be detected by the detector because more light isscattered in the direction of the incident light beam 3 than in otherdirections.

The area illuminated by the lamp 15 through the 7 plates 62, 62 and 63,63' is determined by apertures 65, 65' and 66, 66' in baffles 56, 56 and61,61 respectively and extremities are shown by lines 64, 64, and thearea between the intersection of lines 59,59.

and 64, 64 define the sample volumes 67, 67 from which the scattering ismeasured.

The position of the air inlets and outlets to the chambers is notcritical but in this arrangement the inlets 12, 1 l2 and outlets 13, 13are at the top of the chambers 11, 11 and located either side of thesample volume The baffles 52, 52'; 54, 54'; 55, 55'; are provided toreduce internal reflections within the chambers 11, 11' without takingany part in the definition of the sample volumes 67, 67.

Oblique baffles 68, 68 are provided and function as 12 to chamber 1 isconnected to the exhaust air duct 1 21 from space 20, and outlet 13 isexhausted (to atmosphere if convenient). A blower 22 may be connected toinlet 12 to force air from duct 2 into chamber 1. Inlet 12 to chamber 1is connected to ventilation inlet duct 23, or directly to the outsideatmosphere, again via a blower, 24, if desired. Outlet 13' is alsoexhausted to atmosphere if convenient. Blowers 22 and 24 may be combinedinto a single blower following outlets l3, 13 if negative pressureinside instrument 19 does not give rise to serious inaccuracies due toleakage of air into either chamber.

The block diagram of FIG. shows how clock pulses from Clock PulseGenerator 25 govern the operation of Trigger and Gate Pulse Widthcircuit 26 which in turn causes the High Voltage Discharge Supplycircuit, 27, to discharge through Xenon flash tube 28. The frequency ofpulses (and thus flashes) is not critical; frequency of 1 pulse persecond has been found to be satisfactory but frequencies of 10 persecond may readily be used. The duration of the pulse from the circuit26 is not critical but a pulse duration of 50 us has been found to besatisfactory. Light scattered into Photomultiplier 29 (corresponding toPhotodetector 10 in FIG. 1) causes an output signal which is amplifiedin Amplifier 30 and synchronously detected and stored in SynchronousIntegrator and Hold Circuit, 31. The latter circuit is governed bycircuit 26 so that a charge pro portional to the output from thephotodetector during an interval corresponding to the flashing time ofthe Xenon lamp 28 is stored on a capacitor. This output will include acomponent of noise, and during the following period where the lamp isquiescent, noise from the photodetector is averaged over the wholeperiod and stored in another capacitor in circuit 31. At the end of theperiod, the charges on both capacitors are combined in such a way thatthe average noise charge is subtracted from the charge representing thescattered light signal plus noise, leaving a closer approximation to thescattered light signal than is possible with an uncompensated detector.The resultant signal is fed into Buffer 32, and thence into a firstchannel of a Chart Recorder 33, or a Difference Amplifier 34 (or both).The other input to the difference amplifier, shown diagrammatically at35, is from a second channel (not shown) derived from a photomultipliercorre sponding to the photomultiplier 10 in FIG. 1 and brought through asimilar circuit to that for photomultiplier 29. The output from thesecond channel may be recorded on a second channel of Chart Recorder 33,and the signal from the difference amplifier 34 may be recorded on athird channel of Chart Recorder 33.

A more detailed diagram of one form of the synchronous lntergrate andHold Circuit 31 is shown in FIG. 6. In this circuit a pair of fieldeffect transistors F1 and F2 of opposite channel doping are used tocontrol storage of charge on storage capacitors Cl and C2 respectively.The capacitors Cl and C2 are connected to the output of amplifier 30through resistor R, the time constants RC and RC being much greater thanthe pulse duration to give an integrating effect. The capacitor C storescharge throughout the whole cycle period and thus its charge will berepresentative of the scattering and noise levels. The capacitor C onthe other hand, does not store charge during the flashing period butstores at other times and hence its charge is representative of noise.During the non-sampling period, i.e. the greater part of the cycle, thecapacitors C, and C are effectively in series from the point of view ofthe output and hence their contributions to the output level will besubtracted, and thus the noise is effectively cancelled out.

FIG. 7 shows a series of traces depicting various conditions which mayappear in the output from chart recorder 33, showing voltage on thevertical scale and time on the horizontal. The traces are diagrammaticonly, and in practice the events shown will normally take place over aconsiderable time, with long periods of uniform signal levels beingrecorded between events. Three traces, corresponding to three pens in amultipen chart recorder, are shown: trace A corresponds to the insideair condition monitored in chamber 1: trace B corresponds to the outsideair condition monitored in chamber 1; and trace C corresponds to thedifference Amplifier 34. Obviously. other parameters which may have abearing on the interpretation of these traces may also be recorded, forexample, mains voltage.

If desired, the traces may be recorded in situ, but in manycircumstances it may be desirable to record them at a central monitoringstation serving a number of installations. The signal levels may betransmitted along telephone lines, for example, as tones. thefrequencies of which are respectively related to the amplitudes of thesignals to be recorded.

In FIG. 7, traces A, B and C are all shown as beginning at theirrespective zero levels, indicating clean air inside and outside thespace to be monitored. Events D and E, relatively sharp peaks in themeasured scattering level of the inside air, are typical of thetemporary rises causes by exhalations of smoke by a person smoking acigarette, for example, particularly if he is near the ventilationexhaust duct of the air circulation system. Peak F indicates acalibrating step, which may be the firing of three fusees together eachfusee generating about 20 mg of smoke in the centre of the space beingmonitored. In one working monitoring arrangement, it has been founddesirable to actuate a yellow" alert at this calibrating level. Thisalert is intended merely to draw a supervisors attention to theinstrument, as it does not necessarily indicate a fire. The triggeringlevel may be set at a scattering coefficient [b (scat)] level of about2.2 X 10' m, where b (scat) is the fraction of incident light scatteredby the sample (integrated for all angles between incident and scatteredlight directions). This corresponds to a meteorological visual range ofabout 18 kilometres.

When a dark target is viewed against a light background, the contrastbetween target and background decreases with an increasing scatteringcoefficient between observor and target. When the amounts of light fromthe target and background respectively differ by less than 2 percentthey can no longer be properly distinguished. This happens at the visualrange, which for clear air is approximately 170 km miles) andcorresponds to a b (seat) of about 0.23 X 10 m.

More urgent alert signals may also be designed to operate at higherscattering levels than the yellow. For example, an orange alert,initiating an investigation at the building being monitored, could beset at a b (scat) of 4.4 X 10 m (visual range, about 9 km); and a red"alert, calling the Fire Brigade and operating fire control systems,could be set at a b (scat) of about 7 X 10 m (visual range, about 5.5km). The red alert level, in particular, would have to be adjusted tosuit the particular circumstances of the building being monitored.

Event G denotes the onset of a real fire, and it can be seen that theyellow level (set, for example, at about 1 volt on the recorder) ispassed shortly after the first traces of smoke are generated. Asupervisor looking at the trace could immediately see its steadilyrising form. and take prompt action.

Line H separates the traces for a later incident, shown for convenienceon a much contracted time scale. At point 1, outside smoke levels beginto rise, causing a negative swing in the difference signal. At a laterpoint, J, the inside air begins to show significant signs of increasingscattering caused by smoky air from outside accumulating inside. Thedifference signal returns towards zero, but at point K, where theoutside air has cleared. the difference signal goes positive until thesmoky air inside is cleared. Of course, if a real fire breaks out insideduring this interval of positive excursion, an additional signal, L,will appear and trigger the appropriate alerts.

It will be apparent that many modifications can be made to theembodiment of the invention described herein, and it is to be understoodthat the invention is not limited to the details of the constructionillustrated, but includes all variations falling with its spirit andscope.

We claim:

1. A smoke monitoring instrument having first and second chambers. eachwith a respective photodetector means, a light source common to bothchambers and positioned to simultaneously illuminate part of the interior of each chamber, means for conveying a first air sample from thespace to be monitored into said part of the first chamber, means forconveying a second air sample from outside the space into said part ofthe second chamber, each photodetector means being arranged tosimultaneously receive light from the source which is scattered by itsrespective air sample but not to receive light directly from the source.

2. An instrument as claimed in claim 1 wherein the light sourcecomprises a flash tube and the instrument includes an energizing circuitto operate the flash tube periodically.

3. An instrument as claimed in claim 2 wherein each photodetector meanscomprises a light sensitive element and a detector circuit responsivethereto and operable to produce an output signal representative of theamount of light received by the light sensitive element.

4. An instrument as claimed in claim 3 including means to produce adifferential output signal, being a signal derived from the differenceof the output signals of the detector circuits.

5. An instrument as claimed in claim 3 wherein each detector circuitincludes first and second storages, the first storage being operable tostore a signal representative of the light received by its associatedlight sensitive element during the time the flash tube is operated andduring the time the flash tube is not operated, the second storage beingoperable to store a signal representative of the light received by itsassociated light sensitive element during the time the flash tube is notoperated, said output signal of the detector circuit being derived fromthe difference of the signals stored in the first and second storages,whereby noise signals are reduced.

6. An instrument as claimed in claim 1 wherein the 8. An instrument asclaimed in claim 3 including means to record the output signals of thedetector circuits. i

9. An instrument as claimed in claim 8 wherein'the means to record isoperable to record the difference between the output signals.

10. An instrument as claimed in claim}, including alarm circuitryoperable to produce alarm signals which are dependent upon the outputsignal levels of.

the detector circuit associated with the first chamber. 11. Aninstrument as claimed in claim 8 wherein the means to record is locatedremote from the detector circuits and is coupled thereto by atelecommunications link.

12. An instrument as claimed in claim 3 whereinthe first and secondchambers comprise apair of spaced elongate chambers, the interiors ofwhich are optically black, and said source of light is positionedbetween 1 chambers, each chamber including a window adjacent i to thesource.

13. An instrument as claimed in claim 12 wherein the instrument includesopal glass diffusing surfaces interposed between the windows and thesource.

14. An instrument as claimed in claim 13 wherein the opal glassdiffusing surfaces areplanar and their normals are inclined towardrespective photodetector means.

15. An instrument as claimed in claim 12 wherein the chambers and thediffusing surfaces are mounted upon a removable housing for theinstrument and the source and photodetectors are mounted on a base ofthe instrument.

16. A method of detecting the generation of smoke associated with a firehazard in a space to be monitored, comprising the steps of drawing afirst sample of air from the space to be monitored, drawing a secondsample of air from outside said space, exposing both samplessimultaneously to a source of light and generating two signalssimultaneously, each related to the amount of light scattered by arespective one of said samples.

17. A method as claimed in claim 16 wherein the first sample is drawnfrom an exhaust duct of an aircondi tioning system for the space to bemonitored.

18. A method as claimed in claim 17 wherein the second sample is drawnfrom an inlet duct of the airconditioning system.

19. A method as claimed in claim 16 including the step of storing arecord of the signals over a period of time.

20. A method as claimed in claim 19 wherein the storing of a record isperformed at a location remote from the space to be monitored.

1. A smoke monitoring instrument having first and second chambers, eachwith a respective photodetector means, a light source common to bothchambers and positioned to simultaneously illuminate part of theinterior of each chamber, means for conveying a first air sample fromthe space to be monitored into said part of the first chamber, means forconveying a second air sample from outside the space into said part ofthe second chamber, each photodetector means being arranged tosimultaneously receive light from the source which is scattered by itsrespective air sample but not to receive light directly from the source.2. An instrument as claimed in claim 1 wherein the light sourcecomprises a flash tube and the instrument includes an energizing circuitto operate the flash tube periodically.
 3. An instrument as claimed inclaim 2 wherein each photodetector means comprises a light sensitiveelement and a detector circuit responsive thereto and operable toproduce an output signal representative of the amount of light receivedby the light sensitive element.
 4. An instrument as claimed in claim 3including means to produce a differential output signal, being a signalderived from the difference of the output signals of the detectorcircuits.
 5. An instrument as claimed in claim 3 wherein each detectorcircuit includes first and second storages, the first storage beingoperable to store a signal representative of the light received by itsassociated light sensitive element during the time the flash tube isoperated and during the time the flash tube is not operated, the secondstorage being operable to store a signal representative of the lightreceived by its associated light sensitive element during the time theflash tube is not operated, said output signal of the detector circuitbeing derived from the difference of the signals stored in the first andsecond storages, whereby noise signals are reduced.
 6. An instrument asclaimed in claim 1 wherein the means for conveying a first air samplefrom the space to be monitored is coupled to an exhaust duct of anairconditioning system for the space to be monitored.
 7. An instrumentas claimed in claim 6 wherein the means for conveying a second airsample from outside the space is coupled to an inlet duct of theairconditioning system.
 8. An instrument as claimed in claim 3 includingmeans to record the output signals of the detector circuits.
 9. Aninstrument as claimed in claim 8 wherein the means to record is operableto record the difference between the output signals.
 10. An instrumentas claimed in claim 3, including alarm circuitry operable to producealarm signals which are dependent upon the output signal levels of thedetector circuit associated with the first chamber.
 11. An instrument asclaimed in claim 8 wherein the means to record is located remote fromthe detector circuits and is coupled thereto by a telecommunicationslink.
 12. An instrument as claimed in claim 3 wherein the first andsecond chambers comprise a pair of spaced elongate chambers, theinteriors of which are optically black, and said source of light ispositioned between chambers, each chamber including a window adjacent tothe source.
 13. An instrument as claimed in claim 12 wherein theinstrument includes opal glass diffusing surfaces interposed between thewindows and the source.
 14. An instrument as claimed in claim 13 whereinthe opal glass diffusing surfaces are planar and their normals areinclined toward respective photodetector means.
 15. An instrument asclaimed in claim 12 wherein the chambers and the diffusing surfaces aremounted upon a removable housing for the instrument and the source andphotodetectors are mounted on a base of the instrument.
 16. A method ofdetecting the generation of smoke associated with a fire hazard in aspace to be monitored, comprising the steps of drawing a first sample ofair from the space to be monitored, drawing a second sample of air fromoutside said space, exposing both samples simultaneously to a source oflight and generating two signals simultaneously, each related to theamount of light scattered by a respective one of said samples.
 17. Amethod as claimed in claim 16 wherein the first sample is drawn from anexhaust duct of an airconditioning system for the space to be monitored.18. A method as claimed in claim 17 wherein the second sample is drawnfrom an inlet duct of the airconditioning system.
 19. A method asclaimed in claim 16 including the step of storing a record of thesignals over a period of time.
 20. A method as claimed in claim 19wherein the storing of a record is performed at a location remote fromthe space to be monitored.